fix(journey-editor): unsaved-warning banner + save throws on failure

JourneyEditor now renders UnsavedWarningBanner when showUnsavedWarning
is true. save() wraps onSubmit in try/catch so clearOnSuccess only fires
on success. edit/+page.svelte handleSubmit throws instead of returning
on non-ok responses so JourneyEditor sees the failure.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/geschichte/JourneyEditor.svelte.spec.ts

@@ -4,6 +4,9 @@ import { page, userEvent } from 'vitest/browser';
 import { m } from '$lib/paraglide/messages.js';
 import JourneyEditor from './JourneyEditor.svelte';
 
+vi.mock('$app/navigation', () => ({ beforeNavigate: vi.fn(), goto: vi.fn() }));
+import { beforeNavigate } from '$app/navigation';
+
 const docSummary = (id: string, title: string) => ({
 	id,
 	title,
@@ -612,6 +615,69 @@ describe('JourneyEditor — duplicate document aria-disabled', () => {
 	});
 });
 
+describe('JourneyEditor — unsaved warning banner', () => {
+	function triggerNavigationAttempt() {
+		const calls = vi.mocked(beforeNavigate).mock.calls;
+		if (calls.length === 0) return;
+		const [callback] = calls[calls.length - 1];
+		const cancel = vi.fn();
+		(callback as (nav: { cancel: () => void; to: { url: URL } | null }) => void)({
+			cancel,
+			to: { url: new URL('http://localhost/geschichten') }
+		});
+		return cancel;
+	}
+
+	it('banner is absent before any edit or navigation attempt', async () => {
+		render(JourneyEditor, defaultProps());
+		expect(document.querySelector('[class*="amber"]')).toBeNull();
+	});
+
+	it('banner appears when dirty and a navigation is attempted', async () => {
+		render(JourneyEditor, defaultProps());
+
+		// Mark dirty by editing the title
+		const titleInput = page.getByPlaceholder(/Titel/);
+		await titleInput.element().dispatchEvent(new InputEvent('input', { bubbles: true }));
+
+		// Simulate the user trying to navigate away
+		const cancel = triggerNavigationAttempt();
+		expect(cancel).toHaveBeenCalled();
+
+		await expect.element(page.getByText(m.admin_unsaved_warning())).toBeInTheDocument();
+	});
+
+	it('banner stays after a failed save (clearOnSuccess not called when onSubmit throws)', async () => {
+		const onSubmit = vi.fn().mockRejectedValue(new Error('server error'));
+		render(
+			JourneyEditor,
+			defaultProps({
+				onSubmit,
+				geschichte: makeGeschichte({
+					title: 'Titel',
+					items: [{ id: 'i1', position: 0, document: docSummary('d1', 'Brief A') }]
+				})
+			})
+		);
+
+		// Mark dirty
+		const titleInput = page.getByPlaceholder(/Titel/);
+		await titleInput.element().dispatchEvent(new InputEvent('input', { bubbles: true }));
+
+		// Trigger navigation → banner appears
+		triggerNavigationAttempt();
+		await expect.element(page.getByText(m.admin_unsaved_warning())).toBeInTheDocument();
+
+		// Attempt save — onSubmit throws
+		await userEvent.click(page.getByRole('button', { name: m.geschichte_editor_save_draft() }));
+
+		// Banner must still be visible (isDirty was not cleared)
+		await vi.waitFor(() => {
+			expect(document.querySelector('[class*="amber"]')).toBeTruthy();
+		});
+	});
+});
+
 describe('JourneyEditor — person chips from GeschichteView', () => {
 	it('renders person names in the sidebar chips (PersonView carries no displayName)', async () => {
 		render(